Grab the source code from the JSON.Net library, add it to your solution, and reference it in your projects.
https://github.com/JamesNK/Newtonsoft.Json/tree/master/Src/Newtonsoft.Json
You now have all the features of JSON.Net without any external dependencies.
Then, when you get tired of manually updating it, relax your "only in the framework" rule and add it via Nuget so it's super-easy to manage.
EDIT: There is the two built-in JSON serializers, but they both suck in their own special ways.
JavaScriptSerializer UTC DateTime issues
JavaScriptSerializer, and DataContractJsonSerializer are riddled with bugs. Use json.net instead. Even Microsoft has made this switch in ASP.Net MVC4 and other recent projects.